Halobacteria possess a unique photosynthetic capability which does not depend on the ubiquitous photosynthetic pigment chlorophyll. Instead, photosynthesis in halobacteria depends on a pigment similar to visual pigments (rhodopsins). The pigment has thus been named bacteriorhodopsin. Bacteriorhodopsin is a protein of molecular weight 26,000 which contains covalently bound retinal as the chromophore. It is found in a two dimensional crystalline array in the surface membrane of the bacteria. Bacteriorhodopsin functions as a light deiven proton pump which moves protons from the interior to the exterior of the cell, thus generating an electrochemical potential across the cell membrane. It is thus consistent with the chemiosmotic mechanism proposed for chlorophyll-dependent photosynthesis and oxidative phosphorylation. We have been studying light to biological energy conversion in halobacteria including the mechanism of the proton pump and the coupling of the resulting electrochemical potential to ATP synthesis, ion transport, and other energy requiring processes of the cell. Our objectives are to understand energy coupling in halobacteria since it is a relatively simple system but pertinent to the more general questions of energy coupling in all cells, and to determine the molecular mechanism of the proton pump, since it is probably the simplest example of a membrane pump. Techniques employed include optical spectroscopy of the pigment and optical detection of pH and membrane potentials using indicating dyes, radioactive tracers for the measuring distribution of ions, and general biochemical procedures.